We present a novel approach to proving the absence of timing channels. The idea is to partition the program's execution traces in such a way that each partition component is checked for timing attack resilience by a time complexity analysis and that per-component resilience implies the resilience of the whole program. We construct a partition by splitting the program traces at secret-independent branches. This ensures that any pair of traces with the same public input has a component containing both traces. Crucially, the per-component checks can be normal safety properties expressed in terms of a single execution. Our approach is thus in contrast to prior approaches, such as self-composition, that aim to reason about multiple (k>=2) executions at once.

We formalize the above as an approach called quotient partitioning, generalized to any k-safety property, and prove it to be sound. A key feature of our approach is a demand-driven partitioning strategy that uses a regex-like notion called trails to identify sets of execution traces, particularly those influenced by tainted (or secret) data. We have applied our technique in a prototype implementation tool called Blazer, based on WALA, PPL, and the brics automaton library. We have proved timing-channel freedom of (or synthesized an attack specification for) 24 programs written in Java bytecode, including 6 classic examples from the literature and 6 examples extracted from the DARPA STAC challenge problems.

[ .pdf ]

@inproceedings{antopoulous17blazer,
  author = {Timos Antonopoulos and Paul Gazzillo and Michael Hicks and Eric Koskinen and Tachio Terauchi and Shiyi Wei},
  booktitle = {Proceedings of the {ACM} Conference on Programming Language Design and Implementation (PLDI)},
  title = {Decomposition Instead of Self-Composition for Proving the Absence of Timing Channels},
  month = jun,
  year = 2017
}

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